It is known that nitrogen oxides (NO and NO.sub.2, hereafter NOx) can be reduced to nitrogen N.sub.2 by selective reduction using ammonia (H. Bosch and F. Janssen, Catal. Today, 1988, 369) and thus that these compounds, which are known to contribute to the formation of photochemical fog and acid rain, can be removed from gases discharged into the atmosphere. The reduction of NOx would essentially follow the reactions obeying the following overall equations: EQU NO.sub.2 +4/3NH.sub.3.fwdarw.7/6N.sub.2 +2H.sub.2 O
which take place catalytically. Among the catalysts employed, industry has in particular adopted cubic faujasites (FAU) exchanged with copper (European Patent EP 0,483,201 and U.S. Pat. No. 5,536,483) which have excellent activity in the 250-400.degree. C. temperature window and which are particularly well suited to the treatment of tail gases from most nitric acid plants. Until recently, the presence of nitrogen protoxide (N.sub.2 O) in these discharges was barely a matter of concern, this being a gas deemed to be harmless because it is not involved in the formation of acid rain, until account is taken of its not insignificant contribution to the greenhouse effect. Its removal has thus become a concern of public services and industrial companies. It turns out that the gases treated by SCR using ammonia on most catalysts of the prior art, especially on examples of faujasite Y exchanged with copper, may give rise, in certain temperature windows, to the formation of N.sub.2 O.
It is now known, in particular by the use of the profiles of NOx SCR by NH.sub.3, to distinguish two waves of NO reduction, respectively around 230.degree. C. and above 325.degree. C., and it may be noted in this temperature range that a parasitic reaction of a reduction of NO to nitrogen protoxide N.sub.2 O takes place. It has been possible to correlate this N.sub.2 O formation with a temperature-programmed reduction (TPR) profile, using hydrogen, of the "copper" species in the catalyst, according to a method which was explained in "Characterized Catalysts via Temperature-Programmed Reduction", Chemtech, 1977, 316-302, by J. W. Jenkins, B. D. McNicol and S. D. Robertson, which authors have developed the TPR technique with analysis by a catharometric cell. The corresponding experimental process is developed below in terms of examples.